1. Field of the Invention
The invention relates to an electric oil pump control apparatus for a vehicle, which includes an electric oil pump and a application device that is driven by a hydraulic pressure supplied from the electric oil pump, and with which the response of the application device provided in a shift mechanism to the hydraulic pressure and the useful life of the application device are improved, the invention also relating to an electric oil pump control method.
2. Description of the Related Art
Usually, a vehicle is provided with a shift apparatus that is directly or indirectly connected to an engine and that changes the rotational speed of the engine in a stepwise manner or continuously. An example of such shift apparatus is a stepped automatic transmission. The automatic transmission is formed of multiple planetary gear units, and a desired gear is selected by selectively connecting rotational elements of these planetary gear units to each other. The rotational elements are connected to each other by application devices provided in the automatic transmission. The application devices are driven by a hydraulic pressure. The application devices are applied or released by controlling the hydraulic pressure in an appropriate manner.
The hydraulic pressure of the hydraulic fluid supplied from an oil pump is used as the original pressure, and the original pressure is regulated to a desired hydraulic pressure in a hydraulic pressure control circuit of the shift apparatus. Then, the regulated hydraulic pressure is supplied to these application devices. In many cases, an oil pump is provided in the shift apparatus, and driven in accordance with an operation of the engine.
In recent years, hybrid vehicles in which two types of drive power sources, that is, an engine and an electric motor are used in combination, have come on the market. Because both the engine and the electric motor are used, it is possible to utilize the advantages and make up for the disadvantages of each of the engine and the electric motor. With this structure, hybrid vehicles provide good drivability, that is, the hybrid vehicles are driven smoothly and respond quickly to a control. In addition, the hybrid vehicles consume considerably smaller amount of fuel and emit far less exhaust gases than conventionally powered vehicles. If the shift apparatus is provided in such hybrid vehicles, it may be possible to further enhance the drivability and fuel efficiency. In such hybrid vehicles, the engine efficiency is usually low when the vehicle starts moving, when the vehicle is traveling at a low speed and when the vehicle is traveling at a low torque. In such a case, the engine is stopped and the vehicle travels using the drive power produced by the electric motor.
If only a mechanical oil pump that is driven in accordance with an operation of the engine is provided in the above-described hybrid vehicle, the hydraulic pressure is not supplied when the vehicle travels using the drive power produced by the electric motor. This is because the mechanical oil pump is not driven due to a stop of the engine. Especially, in the hybrid vehicle in which the above-described shift apparatus is provided, an appropriate hydraulic pressure is not supplied to the application devices of the shift apparatus. Therefore, the drive power is not transmitted to drive wheels, which makes it impossible for the vehicle to keep traveling. In order to avoid such inconvenience, in the hybrid vehicle provided with the shift apparatus, an electric oil pump is provided in addition to the mechanical oil pump. When the engine is stopped, the electric oil pump is driven to supply a hydraulic pressure to the application devices of the shift apparatus.
The electric oil pump may be provided not only in the hybrid vehicles but also in other types of vehicles. For example, in a control apparatus for a vehicle described in Japanese Patent Application Publication No. 2000-356148 (JP-A-2000-356148), an electric motor (motor generator) is provided between an engine and a torque converter. When the vehicle is traveling using the power from the electric motor, a hydraulic pressure is supplied from the electric oil pump to the shift apparatus.
When the shift position is in a stop position, for example, Neutral, at which the drive power is not transmitted to drive wheels, the control apparatus for a vehicle described in JP-A-2000-356148 stops the engine to stop the mechanical oil pump and drives the electric oil pump to reliably achieve the hydraulic pressure that is supplied to the shift apparatus. Then, when it is predicted that the vehicle will be placed in a stopped state, the output from the electric oil pump is reduced to reduce the amount of electric power consumed to drive the electric oil pump.
However, when the control apparatus described in JP-A-2000-356148 predicts that the vehicle will be maintained in the stop state and therefore keeps low output from the electric oil pump, if the shift position is changed from the stop position to the cruise position, the hydraulic pressure that is supplied to the application device of the shift apparatus does not rise quickly, and slippage of the application device may occur. As a result, the response of the shift apparatus to the hydraulic pressure may be slow down and the useful life of the shift apparatus may be reduced.